1) Field of the Invention
The present invention relates to a light amplifier and a light amplifying method suitable for use in an optical communication system and, more particularly, to a light amplifier and a light amplifying method suitable for use in a wavelength division multiplexing (WDM) optical communication system and, still more particularly, to a light amplifier and a light amplifying method suitable for use in carrying out a repeater amplification in a wavelength division multiplexing optical communication system.
2) Description of the Related Art
In recent years, a construction of a photonic network using the wavelength division multiplexing transmission technique is in progress for practical use. In the photonic network, by lengthening the distance across which a light signal to be repeated can transmit by being directly amplified without being converted into an electric signal, the cost of the whole optical network is reduced, that is, the cost of the network can be reduced by lengthening the regenerative repeater interval distance. Because of this, a development of lengthening the distance of light amplification repeater transmission is positively in progress.
In other words, in a trunk system network, by lengthening the regenerative repeater (repeater in which after a light signal is once converted into an electric signal and amplified, the electric signal is converted again into a light signal) interval distance, the reduction in cost of the whole network is realized. That is, the cost performance of the construction of a network is determined according to how far the regenerative repeater interval can be lengthened by interposing a function of the repeater, in which a direct light amplification is carried out by a fiber Raman amplifier, an erbium (Er) doped fiber light amplifier, or the like, on the transmission path. It is indispensably necessary to reduce noises in the repeater function for carrying out a direct light amplification in order to realize the extension of the regenerative repeater interval distance as described above.
On the other hand, the trend of the transmission speed per wavelength (channel) toward higher speeds from already existing 10 Gb/s to 40 Gb/s also contributes to the background, which requires that the noises be further reduced in the performance of a light amplification repeater in order to keep up with the trend toward higher transmission speeds as described above. In order to realize the reduction in noise of a light amplification repeater capable of keeping up with the trend toward higher transmission speeds, a technique capable of carrying out a direct light amplification with reduced noises is demanded.
A Raman amplifier described in Patent document 1 shown below is an example that realizes the reduction in noise of a light amplification repeater in order to keep up with the above-mentioned trend toward higher transmission speeds. In the Raman amplifier described in Patent document 1, a Raman amplification optical fiber and a dispersion compensating fiber (DCF) are connected in this order and input light is Raman-amplified in the Raman amplification fiber and at the same time, the dispersion of the Raman amplification optical fiber and a transmission path fiber to which the Raman amplification optical fiber is connected is compensated in the DCF. Then, in order to alleviate the loss produced by the DCF, the Raman amplifier carries out a Raman amplification of the DCF as well as a dispersion compensating fiber Raman amplifier (DCFRA).
In addition to the above, techniques relating to the present invention include those described in Patent documents 2 and 3 shown below.
[Patent document 1] Japanese Patent Laid-Open (Kokai) 2003-131274
[Patent document 2] Japanese Patent Laid-Open (Kokai) HEI 9-179152
[Patent document 3] Japanese Patent Laid-Open (kokai) 2000-98433
However, in the technique described in Patent document 1, a special optical fiber having an effective area of 30 μm2 or less at an exciting light wavelength is used as a Raman amplification optical fiber used for Raman amplification in order to obtain a comparatively high Raman amplification efficiency.
At present, in order to reduce the noise figure (NF) to a level of 3 dB or less in a node constituting a repeater in an optical transmission path, it is necessary to carry out a distributed Raman light amplification of an input side transmission path fiber. In the technique described in Patent document 1 mentioned above, a Raman amplification is not carried out for the input side transmission path fiber, therefore, it is difficult to make the loss of the transmission path fiber appear effectively low and it is also difficult to reduce the NF below 3 dB, which is a theoretical limit that can be realized in an erbium doped optical fiber amplifier (EDFA) (refer to FIG. 5(b) shown in Patent document 1).
Moreover, in the technique described in Patent document 1 mentioned above, if an attempt is made to carry out the distributed Raman amplification of the input side transmission path fiber for the purpose of obtaining the gain obtained by the Raman amplification in the Raman amplification optical fiber used for Raman amplification, in addition to the purpose of making the loss of the transmission path fiber appear low, it becomes necessary to use an extremely large exiting light power equal to or greater than about 500 mW.
If it is intended to connect an exciting light source and an optical fiber with an easily-detachable connector in a configuration for supplying exciting light to be used as a DRA to an optical fiber, it is preferable, at present, to avoid the application of exciting light having such an extremely large power from the standpoint of the convenience of handling. In IEC61292-4TR (IEC/TR 61292-4: OPTICAL AMPLIFIERS —Part 4: Maximum permissible optical power for the damage-free and safe use of optical amplifiers, including Raman amplifiers), cautions are described in the case where exciting light having such an extremely large power is applied to a DRA. Therefore, without employing the foregoing extremely large power as an input power to a transmission path of the exciting light to be applied to the DRA, the measures to reduce noises are required.
It is needless to say that the above-mentioned technique for a direct light amplification is required to reduce noises on the assumption that the light level at the stage of being sent out to a transmission optical fiber for repeater is amplified up to a level required by the optical communication system.
Moreover, the techniques described in Patent documents 2 and 3 mentioned above also are not for disclosing a technique that realizes the gain and the noise characteristic of a light amplification repeater in accordance with the trend toward higher transmission speeds as described above.